The present invention relates to integrated imaging sensor circuits.
1. Background Converter Calibration
Optical arrays for scanning or imaging applications are subject to variations in response due to non-uniform external optics and illumination. In order to prevent clipping of signals during analog-to-digital conversion, the full scale of the analog-to-digital converter (xe2x80x9cADCxe2x80x9d) must be greater than the maximum output of the most responsive pixel in the array. (This is usually set by controlling light intensity and/or light integration periods.) However, since there can be a substantial variation in responsivity between the most responsive pixels and the least responsive pixels of the array, the less responsive pixels will have lower dynamic range and higher quantization noise than the more responsive pixels. Other prior-art solutions use a higher precision external ADC, and other digital techniques to restore the dynamic range. The problem is exacerbated by stray background light which raises the dark level (lower limit) of the pixel output, further reducing dynamic range and increasing quantization noise.
Another source of difficulty is the fall-off in brightness at the edge of a lensed image. Any lens of a given focal length and f-number will have uniform brightness only over a limited image radius, and other parts will be less bright. In many applications this is not critical, but it is still a significant effect.
2. Optical Sensor Array With Zone-Programmable Gain and Offset
The present application discloses a technique where gain and offset of particular pixel zones may be programmed prior to analog-to-digital conversion. Since the variations in responsivity due to external optics and illumination vary gradually as a function of geometry, the problem of reduced dynamic range and increased quantization noise can be substantially reduced by programming the respective gain and offset of groups of adjacent pixels. In this way, the dark and white levels of the groups (zones) correspond closely to the zero and full scale of the analog-to-digital converter (xe2x80x9cADCxe2x80x9d) being used to convert the pixel signals. Thus, the converter input is closely matched to the dynamic range of the pixel outputs. The innovative circuit comprises a control circuit capable of addressing all pixel zones. A register file contains digital words for controlling a programmable gain amplifier and an offset digital-to-analog converter (xe2x80x9cDACxe2x80x9d). The offset DAC generates an offset voltage in response to a command from the register file. The offset voltage is then summed with a serial analog output of the pixel array. The output of the programmable amplifier is then digitized and sent to an I/O interface circuit for access to external circuits. In one class of embodiments the zone-programmable gain and offset parameters are adjusted dynamically, so that the sensing parameters can be optimized with respect to overall scene brightness as the viewed scene changes.
A principal advantage is that when adjusting less responsive pixels prior to conversion, quantization noise becomes less of a concern, and less down-stream signal processing is required than in prior-art designs. Various of these innovations also reduce quantization noise in pixels with lower response characteristics.